Embodiments of the present invention relate to methods and apparatus for removing contaminants from water. More particularly, embodiments of the present invention create arsenic-adsorbing or other contaminant-adsorbing enhanced iron III hydroxides or iron hydroxycarbonates that remove arsenic and other contaminants from anoxic or oxic water using continuous in-stream and batch process methods which may combine both dissolved inorganic arsenic V and arsenic III species with minimal iron salts at elevated solution pressures, some times with forced air injection or aeration, followed by depressurization of the solution. Including pressurization and subsequent solution depressurization within the treatment sequence rapidly increases particulation reaction velocities involving dissolved inorganic arsenic V and arsenic III species, transition and post-transition-metal contaminants, other contaminants and the injected iron salts. Aeration at ambient conditions, air injection after pressurization and addition of chemical oxidants may also help increase reaction rates in certain solutions. Subsequent separation of the particulated species from the solution can render the water suitable for human consumption.
The present invention also reduces the amount of iron needed for the removal of contaminants when compared with processes carried out completely at ambient pressure.
Domestic water supplies often come from underground aquifers which contain anoxic water that has leached through and otherwise contacted minerals, sediments and rock layers for extended periods of time. These minerals, sediments and rocks often contain high concentrations of minerals, metals and other elements and compounds that are deleterious to human health. As a consequence of this contact, water in these aquifers becomes contaminated with some of the indigenous contaminants rendering the water unsafe for human consumption. Other water sources that may be used for domestic water supplies may also become contaminated with metallic ions and other contaminants through industrial pollution and other processes. These contaminated waters, prior to human consumption, will require remediation treatments.
Arsenic is one element that is often found in water sources and is pathological, in terms of human health of specific population segments, at all levels of concentration in drinking water concentrations. Several methods are known for removing arsenic species from water, however each has limitations and or disadvantages, which make embodiments of the present invention preferable in many applications.
Methods using electrolytic cells for electrochemical insolubilization of metallic ions using sacrificial anodes are known for arsenic removal, but require an energy source to power the electrolytic cell and do not address the issue of removing dissolved inorganic arsenic III species.
Other known methods require significant changes in pH levels to the bulk solution to effectuate precipitation of arsenic and other contaminant species from solution (i.e. lime addition). These processes additionally require a readjustment of the pH level after treatment to near-neutral conditions. These two requirements are chemically intensive and equipment intensive. Another method requires ultra-filtration and the addition of anti-scalants as pre-treatments for reverse osmosis treatment systems that remove arsenic species. This treatment method is ineffectual in terms of removing dissolved inorganic arsenic III species without chemical oxidation, which in turn is deleterious to the systems membranes. Other methods utilize adsorptive materials that are rapidly consumed and become solid waste along with the regenerative solutions required by the system (i.e. active alumina, ion exchange).
In some instances, a treatment method will efficiently remove dissolved arsenic and other species from a specific water source but be far less efficient when the same operational parameters are applied at a second autonomous water source. In particular, inconsistency of performance capabilities of the iron coagulation treatment system is problematic.
To achieve efficiency with nearly all anoxic ground water sources this method often requires large additions of either ferric chloride or ferric sulfate to the bulk solution. Some of these water sources require the addition of nearly 40-mg of ferric chloride per liter of treated water to achieve a residual arsenic level less than 5 to 10 parts per billion. This chemically-intensive practice of adding voluminous ferric chloride solutions results in a significant pH shift in the bulk solution towards the acid region. To counter the acid shift, the treatment facility is engineered to add base solutions to the bulk solution causing a return of the bulk solution pH to near-neutral conditions. After physical separation of the newly created particulate matter from the bulk solution the resulting accumulation of solid waste is massive, which is problematic in terms of disposal requirements.
Many known methods of arsenic removal also fail to reduce arsenic concentrations to acceptable levels. Some methods only reduce arsenic concentration to a level of approximately 20 to 50 parts per billion. This may meet some current standards, however standards are likely to become more stringent in the future rendering these methods obsolete and unusable. Furthermore, the increased protection provided by methods that significantly reduce contaminant levels in drinking waters is a benefit to consumers as well as water suppliers.
Most of these existing methods are overly complex, labor intensive, produce large waste streams, require large facilities and land, are expensive and require the addition of large quantities of various chemicals for precipitation and pH adjustment.
What is needed is a method and physical system that is conventional in physical design and construction that provides consistent and near-complete removal of all contaminating arsenic species, heavy-metal, transition-metal and post-transition-metal contaminants contained within all types of water sources and yields a small solid waste product.
The methods and apparatus of embodiments of the present invention provide for the removal or reduction of dissolved inorganic arsenic V and arsenic III species as well as other metallic and nonmetallic contaminants in anoxic or oxic aqueous solutions through continuous in-stream and batch method processes. Under preferred methods of the present invention, an anoxic or oxic aqueous solution may be treated by the addition of iron salts to the solution combined with pressurization and depressurization of the solution. The processes of embodiments of the present invention may occur at a substantially neutral pH.
Chemical oxidants, prior to pressurization, may also be added to speed-up or otherwise enhance the reaction. The iron-doped solution may also be aerated prior to pressurization. In the case of an arsenic contaminated solution, salt addition and optional oxidant addition and aeration are followed by pressurization of the solution to cause dissolved inorganic arsenic V species to precipitate and stabilize. Pressurization prepares dissolved inorganic arsenic III species to become particulate during the subsequent depressurization and ambient reaction. Additional aeration, air injection or other oxygenation methods may be performed prior to or during pressurization. The pressurization step of a preferred treatment method lasts for a short duration of time, typically 1 second to 60 seconds, followed by a depressurization step to near ambient pressures for typically 1 to 5 minutes. However, for some methods and solutions, pressurization step reaction times of 2 minutes or more may be used and a depressurization step of as little as 5 seconds to as much as 45 minutes may be used.
The depressurized aqueous solution may reside in an ambient pressure reaction vessel or conduit in a quiescent state (batch process) or free-flowing state (continuous in-stream) for a period of time prior to physical separation. Particulate and precipitated arsenic-containing or other contaminant-containing solids are stabilized and are then separated from the solution by pressure filtration, sedimentation or other solid-liquid separation methods.
Arsenic removal efficiencies, based upon efficient filtration capabilities for physically removing particulate matter at 5 micron in size and greater, are such that final effluent concentrations are typically less than 2 parts per billion (ppb) arsenic. During continuous in-flow processes, pressurization may be achieved by pumping the solution into a pressure tank or by utilizing the head pressure of pumps associated with ground water sources to pump directly into pressure tanks. An inverted siphon or discharge into a tank of sufficient depth to achieve the desired pressure are other methods of achieving pressurization during in-flow processes.
One of the advantages of selected methods of the present invention is their flexibility. These methods may be applied to large or small scale batch methods for arsenic and other contaminant removal. For batch processes, the solution may be pressurized by filling a tank with iron-doped or iron-oxidant-doped solution, which may also be aerated, followed by pressurization by electric pump or other means. After a short reaction time under pressure has elapsed, the solution is then depressurized to allow for the final particulation reactions to occur. Similar arsenic-removal benefits may be achieved by injection of a solution of iron salt at elevated pressure, followed by precipitation at ambient pressure. In a small-scale operation, pressurization may be achieved by the use of a hand pump. Pressurization levels between 10 psi and 120 psi have been found to significantly reduce both dissolved inorganic arsenic V and III species levels in anoxic and oxic aqueous solutions, however pressures between about 30 psi and about 60 psi are preferred for solutions with typical arsenic contamination. Accordingly, reaction velocities involving the particulation of the dissolved inorganic arsenic species may be dependent upon the pressure applied during the pressurization step. The less pressure utilized the slower the reaction kinetics and less efficient the overall removal of the dissolved arsenic III species. Overall removal of the indigenous dissolved inorganic arsenic V species is not adversely effected.
These processes may even be used for emergency water treatment in small, hand-pumped, pressure tanks. Likewise, these methods may be used for large-scale water treatment operations where hundreds of cubic feet per minute are treated. Elevated pressurization of the treated aqueous solution, followed by a period of time under ambient pressure, vastly accelerates the reaction kinetics of the precipitation-and-stabilization reaction within a treatment sequence involving pressure as an initial condition followed by depressurization to ambient pressure or within a treatment sequence including ambient pressure as an initial condition followed by pressurization then followed by depressurization to ambient pressure.
Accordingly, it is an object of some embodiments of the present invention to provide methods and apparatus for reducing the concentration of arsenic species, metallic ions, transition-metal and post-transition-metal elements, and ions and complexes in an aqueous solution.
It is another object of some embodiments of the present invention to provide methods and apparatus for reducing the concentration of both dissolved inorganic arsenic V and arsenic III species in an anoxic or oxic aqueous solution.
It is also an object of some embodiments of the present invention to provide methods and apparatus for improving the quality and potability of a water supply.
Another object of some embodiments of the present invention is to provide methods and apparatus for continuous in-stream reduction of the concentration of both dissolved inorganic arsenic V and arsenic III species in an anoxic or oxic aqueous solution.
A further object of some embodiments of the present invention is to provide methods and apparatus for batch-process reduction of the concentration of both dissolved inorganic arsenic V and arsenic III species in an anoxic or oxic aqueous solution.
A further object of some embodiments of the present invention is to provide methods and apparatus for continuous and batch-process reduction of the concentration of Sb, Se, Ba, Ag, Tl, Zn, Cd, Cr, Cu, Pb, Mn, Hg, Mo, Ni, and PO4xe2x88x923 from aqueous solution.
These and other objects and features of the present invention will become more fully apparent from the following, description and appended claims, or may be learned by the practice of the invention as set forth hereinafter.